Method of injection molding of tubular parts to eliminate weld lines

ABSTRACT

Injection molding method removing weld lines from a part, such as a parison, which weld lines are caused by the interruption or splitting of the flow of plastic material around core support pins or the like by the provision of an annular mixing chamber which opens into the plastic material flow passage. The plastic material is caused to move radially into the mixing chamber where it is spread by reason of the increasing volume encountered, accompanied by a reversal of flow radially inwardly into an annular injection gate. The movement of plastic material within the chamber and the resultant increase in temperature by reason of shear, eliminates the weld lines which have been formed in the material by reason of the splitting of flow around the pins.

This is a continuation of Ser. No. 104,606 filed Nov. 18, 1987 nowabandoned which is a division of Ser. No. 886,459 filed July 17, 1986,now U.S. Pat. No. 4,711,602 issued Dec. 8, 1987.

BACKGROUND OF THE INVENTION

It has long been recognized that obstructions upstream of a gate, in themolding of tubular parts, such as parisons, about a core or pin, canresult in flow lines in the part. In the case of a parison, the flowlines are carried to the blown part. Such flow lines are caused by theparting of the plastic material, such as a polymer, as it flows aroundan obstruction, such as a core-support pin, positioned upstream of theorifice or gate. At the least, flow lines result in poor appearance ofthe blown piece. However, such lines can severely impair the strength ofa finished part. Strength impairment is particularly critical in theforming of parisons from thermoplastic polymers which have high meltviscosities and which, when parted, do not readily flow or mix together,or rejoin, in a laminar state. While the finished part may present onlya slight marring when physically examined, with many such polymermaterials it has been found that the finished part is also severelyweakened along these lines and can lead to premature failure. Suchpremature failure is particularly critical in blown automotive bellowsand boots which are subject to mechanical as well as environmentalstresses.

The problem is particularly acute in the manufacture of boots andbellows and the like from parisons which are formed about cores, wherethe latter are supported by conventional radially extending core supportpins at the inlet end of the mold. There accordingly exists a need forapparatus and method for the remixing of such polymer material on thedownstream side of such core support pins to eliminate such weld linesand lines of imperfection or weakness in the completed part.

SUMMARY OF THE INVENTION

This invention relates to the molding of tubular parts and moreparticularly to the molding of parisons for use in the blow molding offinished parts, such as boots and bellows for automotive applicationsand the like. More particularly, the invention relates to theelimination of weld line problems caused by upstream obstructions in thepolymer passageway, such as core support pins.

It is believed that the sensitivity of certain polymers to reform orreweld after having been separated by flow around a core pin or the likeis due at least in part to the chilling or surface cooling of thepolymer, such that it does not readily reweld downstream of the pin. Asan example, while the polymer may be injected at 450° F., the metalmaking up the mold and mold parts may be substantially cooler, such as340°, thus providing a chilling effect to the surface of the materialwhere it has been parted.

It has been found that the material may be remixed to eliminate the weldline or line of discontinuity by causing the material to undergo shear,such as by causing the material to be folded back upon itself. The addedturbulence and resulting heat dissipates the relative chill at thecontact line, and permits the remixing of the polymer across the lineboundaries, thereby rewelding the material as a homogeneous mass.

More particularly, the invention is adapted to the manufacture ofparisons, to eliminate weld lines which are caused by the interruptionor splitting of the flow of the plastic melt, between the injector andthe gate opening into the parison cavity, by reason of one or moreinterposed core support pins which extend through the polymer passagewayleading to the gate, upstream of the gate. A generally radially formedremixing chamber communicates at its inner circumference with the gateand with a generally radially outwardly directed flow passage downstreamof the core support pins. The plastic material flows outwardly into thechamber into an expanding volume, where the flow is reversed accompaniedby radially inward flow through into a contracting volume leading to thegate. The resulting shear has a tendency to increase the temperature ofthe polymer, and the resulting mixing by reason of the flow of theplastic first radially outwardly into the mixing chamber and thenradially inwardly to the gate, effectively remixes the material acrossthe weld lines so that the material entering the parison cavity throughthe gate is circumferentially homogeneous.

It is accordingly an important object of this invention to provide forthe remixing of plastic material leading to an extrusion gate, in themanufacture of a parison.

Another object of the invention is the provision of injection apparatusand method in which a generally radially directed remixing chamber ispositioned inwardly of an annular gate to a parison cavity.

Another object of the invention is the provision of molding apparatus inwhich weld seams otherwise caused by the division of flow of a polymermaterial to a cavity are eliminated by remixing.

A still further object of the invention is the remixing of plasticmaterial which has been flow divided, in a generally radially arrangedcavity, for delivery of such remixed material to a parison cavitythrough a gate free of weld lines or lines or weaknesses.

These and other objects and advantages of the invention will be apparentfrom the following description, the accompanying drawings, and theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary sectional view through a die head for theinjection molding of parisons, incorporating a mixing chamber inaccordance with this invention; and

FIG. 2 is a transverse section looking generally along the line 2--2 ofFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a center fed or mold die head is illustratedgenerally at 10, as supporting a core pin 12, which core pin extendsaxially from the head 10 into the interior of a mold 15, for theextrusion or injection forming of a parison 20. The die head 10 isformed with a central melt passageway 22, which passageway is divided bythe rear conical portion 24 of the core pin 12 into a circumferentiallyannular diverging passageway section 25 and to a cylindrical annularpassageway section 26.

The core pin 12 itself is conventionally supported within the mold 15 ongenerally radially extending core support pins 30. The core support pins30, which may be four in number, are shown in FIG. 2 as supporting thecore pin generally centrally of the passageway 26 and centrally of themold cavity 32 defined between the body of the core pin 12 and the mold15. It will be understood that the central or inlet passage 22 is fed bya polymer resin or melt through a suitable adapter from a plasticinjection machine, to form the parison 20. Center-fed molds of the kindshown are in common use since they have the advantage of providing arather uniform distribution of the plastic melt around the core.However, the flow of polymer material is caused to be divided as itflows around the core support pins 30 and to the downstream side of thepins as defined by the passageway section 26.

The invention provides apparatus which causes the remixing of thepolymer material downstream of the core pins 30 to eliminate the weldline which would otherwise be formed in the polymer material, aspreviously noted. For this purpose a generally radially-extending mixingchamber 40 is formed in the head 10 between the passageway 26 and anaxially tapered gate 42 leading into the mold 15. The chamber 40 isconnected to the passageway portion 26 through a radially out-turnedsection 45, which directs the polymer material radially outwardly intothe cavity 40. As shown in FIG. 1, the mixing chamber defined by thecavity 40 is annular and is generally rectangular in cross section, andcommunicates with the polymer at an inlet region 47 at the outer end ofthe radial section 45 and communicates with the gate 42 at an outletportion 48 axially spaced from the inlet port or region 47.

It will be noted that the mixing chamber 40 has a volume which is atleast twice the volume of the flow channel defined by the passagewaysection 26 downstream of the impediments or pins 30. Further, it definesan offset which at least equals the radial dimensions of the channelportions 25 and 26, and in effect provides a region of expanding volumeas the plastic material enters through the entrance region 47 and flowsradially outwardly to the outer wall 50. The direction of movement ofthe material is reversed within the chamber 40 as the material proceedsto the outlet region 48 and to the gate 42 for injection into the moldcavity 32 to form the parison 20. The expanding volume of the chamber 40in the radially outward direction, and the corresponding contractingvolume occupied by the polymer material as its direction of flowreverses and moves radially inwardly effectively remixes the materialacross the weld lines or lines of parting, caused by the core pins 30.Further, the movement within the chamber 40 induces frictional heat intothe material by reason of the shear on the material which overcomes theresistance of the material to reweld and reform across the shear linesotherwise caused by the chilling at the pins 30. When the material exitsthe chamber 40 through the region 48 and gate 42, it has been thoroughlyremixed and no weld lines are visible thereafter.

While the method herein described, and the form of apparatus forcarrying this method into effect, constitute preferred embodiments ofthis invention, it is to be understood that the invention is not limitedto this precise method and form of apparatus, and that changes may bemade in either without departing from the scope of the invention, whichis defined in the appended claims.

What is claimed is:
 1. In the delivery of molten plastic material to amold in the manufacture of tubular plastic parisons in which the plasticmaterial is delivered to the mold through a passageway, and in which thematerial is parted by an flow-dividing impediment in the passageway, themethod of mixing said material downstream of said impediment toeliminate the weld line otherwise formed in the products by theimpediment, comprising the steps of causing said material downstream ofsaid impediment, comprising the steps of causing siad materialdownstream of said impediment and prior to delivery thereof to said moldto move in a direction generally radially outwardly into an annularmixing chamber which has increasing internal volume with increasingradial dimensions, causing said material to reverse direction in saidchamber and flow radially inwardly upon itself while within said chamberto a constricted exit region at a radially inner portion of saidchamber, whereby the movement of said material into an increasing volumefollowed by radial inflow upon itself induces frictional heat and causesremixing of said material across the weld line formed by theflow-dividing impediment.
 2. The method of claim 1 in which saidmaterial is caused to move into said chamber which has at least twicethe volume defined by the passageway immediately downstream of saidimpediment.